Molecular dynamics simulations have been performed for a large-scale system consisting of 400 000 atoms of liquid metal Al. To describe the complex microstructural evolutions in the liquid system during the rapid cooling processes, the tracing atom method and cluster bond-type index method have been used. It is demonstrated that the number of (12 0 12 0) icosahedral clusters, consisting of the 1551 bond type, with a higher degree of ordering, increases continuously and plays a critical and leading role in the solidifying transition. Various cluster configurations, formed by icosahedral clusters and Frank-Kasper, Bernal and defective polyhedra, produce the short-range-order regions in this amorphous system, while the atoms not taking part in forming clusters give the sparse regions possessing disorder characteristics. Large cluster configurations consisting of more than 150 atoms have been found and are shown to be formed by combining smaller clusters and to be different from those obtained by gaseous deposition and ionic spray methods.